A shift control system has been known that switches, according to operation of a shift lever by a driver, the shift range of an automatic transmission under electrical control. As such a shift control system, a system has been proposed that stores in a nonvolatile memory a rotational position of an actuator as well as a shift range of an automatic transmission prior to powering-off, in order that the absolute position of the actuator may be known when the power is turned on again after the powering-off (see Japanese Patent Laying-Open No. 2002-323127 for example). A shift-by-wire system has also been proposed that uses a value of a potentiometer that indicates a drive amount of an actuator for accurately positioning a manual valve (see Japanese Patent Laying-Open No. 2002-349702 for example).
Japanese Patent Laying-Open Nos. 2002-323127 and 2002-349702 have a precondition that the absolute position is known. Therefore, if acquired information about the position of the actuator is merely a relative amount of change in rotor position that is detected from an encoder output, the actuator cannot appropriately be controlled. Consequently, rotation of the actuator exerts some load on a shift switching mechanism, leading to deterioration in durability of the shift switching mechanism. If the shift switching mechanism is designed with the aim of improving the durability, the shift switching mechanism increases in size as well as cost.
Japanese Patent Laying-Open No. 2002-310294 discloses a shift range switching device of an automatic transmission that has an easy-to-mount actuator and accurately makes a switch between shift ranges. The shift range switching device includes a shift range switching mechanism for switching a shift range of the automatic transmission to one of various drive ranges including the parking range, the actuator functioning as a power source of the shift range switching mechanism, a control circuit driving the actuator according to a switch instruction which is input through an external operation to control the shift range so that the shift range of the automatic transmission is switched to a shift range according to the switch instruction, and a detection circuit detecting the shift range position of the automatic transmission to send a detection signal to the control circuit. The control circuit sets a reference value of the shift range position of the automatic transmission from the detection signal of the detection circuit to control the actuator based on the reference value.
The shift range switching device of the automatic transmission disclosed in Japanese Patent Laying-Open No. 2002-310294 drives the actuator according to the switch instruction which is input through an external operation to control the shift range of the automatic transmission so that the shift range is switched to the one following the switch instruction. From the detection signal of the detection circuit which detects the shift range position of the automatic transmission, the reference value of the shift range position of the automatic transmission is set and, based on this reference value, the actuator is controlled. Thus, regardless of the positional precision with which the shift range switching mechanism is mounted as well as differences between individual actuators, no step of adjusting the angle at which the actuator is mounted on the automatic transmission is necessary and accordingly, mounting of the actuator on the automatic transmission is facilitated. In this way, mounting and maintenance of the actuator are facilitated and the shift range of the automatic transmission can accurately be switched by driving the shift range switching mechanism.
Although the shift range switching device of the automatic transmission disclosed in Japanese Patent Laying-Open No. 2002-310294 has the advantage of the facilitated mounting and maintenance of the actuator, the shift range switching device is required to calculate a count value which indicates the center of each range, by referring to a count value of a counter from an output signal of the detection circuit (neutral start switch) which detects the shift range position of the automatic transmission. Based on the reference value which is set according to the count value indicative of each range, the actuator is controlled. Accordingly, regardless of differences between individual actuators, the step of adjusting the angle at which the actuator is mounted on the automatic transmission is unnecessary to facilitate mounting of the actuator on the automatic transmission. Then, the neutral start switch is indispensable, resulting in problems of increases in weight and cost due to the presence of the neutral switch.